The researchers now are testing their lead molecule in
mouse xenograft models for prostate cancer. If the results are promising, MSKCC
could consider running a clinical trial.

Antiandrogen drugs are one of the standard treatment
options for prostate cancer, but many patients progress to castration-resistant
disease in 12-18 months.2
Acquired mutations in the androgen receptor (AR) are one of the key
events that can cause resistance to antiandrogen drugs.3-5

Xtandi is a second-generation oral androgen receptor
antagonist that received FDA
approval in August 2012 to treat metastatic castration-resistant prostate
cancer (CRPC) in patients who previously received docetaxel.
Last month, the EMA's
Committee for Medicinal Products for Human Use recommended approval of an MAA
for Xtandi.

In the U.S., clinicians already are seeing patients whose
tumors initially respond to Xtandi but then develop resistance.

"We wanted to know how this could happen," said
Charles Sawyers, chair of the Human Oncology and Pathogenesis Program at MSKCC
and an investigator at the Howard Hughes Medical Institute.
"We suspected that mutations in the androgen receptor might be one
mechanism and designed a screen to look specifically for them."

The researchers developed a mutagenesis screen to identify
cells with mutations in AR that confer resistance to a particular drug. When
applied to Xtandi, the screen implicated the F876L point mutation in AR as the
dominant mutation that confers resistance to the drug.

In a series of validation studies using Xtandi-sensitive
prostate cancer cell lines and mouse xenograft models, the researchers found
that the F876L mutant AR could spontaneously emerge following prolonged
exposure to Xtandi or ARN-509, a second-generation
AR antagonist from Aragon Pharmaceuticals Inc.
that is in Phase II testing for CRPC.

Xtandi and ARN-509 are both bisaryl-thiohydantoin
AR antagonists. Sawyers co-discovered both molecules and is a cofounder of
Aragon.

Follow-up structural modeling studies and in vitro
assays showed that Xtandi and ARN-509 both antagonized wild-type AR but actually
activated the F876L mutant receptor. Not surprisingly, both were ineffective at
killing prostate cancer cells that expressed the mutant receptor.

The researchers used the insights gained from these assays
and structural modeling studies to design and synthesize analogs from the
Xtandi scaffold that have an additional cyclic hydrocarbon ring-dubbed the
D-ring-attached to the molecule's central B-ring.

In prostate cancer cell lines, three of the resulting
D-ring-substituted molecules from the series inhibited growth of prostate
cancers that expressed the F876L mutant AR, whereas Xtandi did not.

"The key finding is the discovery of a mutation in AR
that causes resistance to enzalutamide," said Sawyers, the
co-corresponding author. "Now that we know about it, we can start to look
for it in patients. We also showed that certain chemical modifications can be made
to enzalutamide that overcome the drug resistance."

"This work paves the way for the development of a new
series of antiandrogens and further supports the case that the androgen
receptor could remain a critical and valid target even in patients who have
late-stage disease," said Jeffrey Hager, senior director of biology at
Aragon. "The data from this study suggest that among the patients that
progress after initially responding to enzalutamide or ARN-509, there exists a
subset who have tumors that remain fully addicted to the androgen receptor."

Hager said the findings also improve the field's
understanding of ligand-specific mutations in AR by providing insights on how
such a mutation can affect the activity of a given class of antiandrogens and
how to modify such molecules to circumvent the mutation. "The androgen
receptor is known to acquire ligand-selective mutations that can flip a
molecule from being an antagonist into an agonist," he said.

Importantly, Hager added that such mutations do not typically
impart agonist activity across all chemical classes of antiandrogens. Indeed, in
vitro cellular assay data showed that structurally distinct
first-generation AR antagonists such as Casodex bicalutamide and the
generic hydroxyflutamide did not activate F876L mutant
AR.

The assay also showed that mutant ARs activated by Casodex
or hydroxyflutamide were not activated by Xtandi and ARN-509.

The next step is determining the relevance
of the F876L mutation in a clinical setting.

"It will be important to determine whether the
identified mutation is found in patients who are being treated with
enzalutamide or ARN-509 and if so the frequency at which it occurs," said
Hager. "The latter will have a strong influence on how excited drug
developers such as Aragon will be to develop compounds that can circumvent the
resistance mutation."

Hager added that any compounds that do block the mutant
receptor could be accompanied by a companion diagnostic to identify patients
who are most likely to respond and to guide treatment decisions. He also said
it would be a good idea to do sequencing studies on tumor samples from patients
receiving second-generation antiandrogens to determine if there are mutations
in AR that can spontaneously arise in the clinic but not in preclinical models.

"For such mutations, one will then need to go back to
preclinical studies to elucidate how they affect the activity and function of
the androgen receptor," he told SciBX.

Aragon has an ongoing internal program focused on
understanding potential resistance mechanisms against second-generation
antiandrogens such as ARN-509 and Xtandi, which parallels the work of Sawyers'
group. Hager said the company is not disclosing additional details.

Meanwhile, Sawyers' group at MSKCC is evaluating DR103,
the lead D-ring-substituted AR antagonist identified from the study, in mouse
xenograft models. "If the results from these studies look promising, we
will consider scaling up for more preclinical testing and a possible clinical
trial at MSKCC," he told SciBX.

MSKCC has filed two provisional patent applications
covering the AR mutation and the new chemical entities described in the eLife
paper. MSKCC declined to disclose licensing details.

Access this BioCentury Innovations article Cover Story for your individual use via a permanent link that allows you to read or print the article, and any sub-articles, charts, tables and/or graphs related to this specific story: $50.
The article link will be posted on the purchase transaction web page, and also emailed to you with your purchase confirmation.

Purchase This Article for Limited One-Time Distribution and Posting to Your Website :

Receive a formatted PDF reprint of this article, including any sub-articles, charts, tables and/or graphs related to this specific article, with rights for limited one-time redistribution and posting to your website: $750. Please allow 24-48 hours for delivery.

Purchase Options

Purchase this article for individual use $50 USDPurchase this article for limited one-time distribution and website posting $750 USD